The implementation of laser diodes systems for most applications involves the joining together of a semiconductor substrate containing the laser diode(s) or a laser diode arrays to a metallic substrate. This implementation approach has always involved the use of an intermediate layer of solder between the two substrates to provide sufficient adhesive strength for practical applications.
The joining or mating together of metal substrates to laser diode or laser diode array semiconductor substrates is performed for one or more of the following reasons. First, to provide increased mechanical stiffness and strength to fragile semiconductor substrates, which are typically made from single crystal materials. Second, to provide one or more electrical connections to the semiconductor substrate, so as to allow electrical currents and voltages to be applied to the laser diode or laser diode array devices for them to operate.
Third, to provide the ability to move heat away from the semiconductor device(s), which typically heat up very significantly during operation. Specifically, the metal substrate functions as a thermal heat sink or thermal heat spreader that facilitates the removal of heat from the laser diodes made in the semiconductor substrate. The metal heat sink may employ cooling fins, channels and/or other fluid handling structural shapes or elements to facilitate an increased heat transfer rate from the metal to a cooling fluid or cooling device.
Fourth, to facilitate the packaging of the laser diode device(s), and thereby, (1) protect the semiconductor laser diode device(s) from the environment, (2) facilitate the electrical connection to the semiconductor laser diode device(s), (3) facilitate the optical connection to the semiconductor laser diode device(s), (4) protect the semiconductor laser diode device(s) from damage during use and handling, and/or, (5) keep the semiconductor laser diode device(s) clean from dust and other airborne particulate matter.
The use of an intermediate layer or layers, such as solders, between a metal substrate and a semiconductor substrate, as currently practiced, can have many disadvantages and shortcomings.
The use of intermediate layer(s) made from the solders and adhesives that are commonly used for joining laser diode semiconductor substrates to metal substrates can result in large “built-in” residual stresses that can have detrimental effects on the performance of the semiconductor laser diode devices. The differing thermal expansion coefficients of the metal and semiconductor substrates, combined with the elevated temperatures required to perform the soldering process, as well as thermal stresses resulting from the heating of the laser diodes when operating, can result in significant built-in stresses between the mated substrates once they are cooled to room temperature, as well as during operation.
These built-in stresses can cause the bandgaps and energy states in the semiconductor material to be altered, thereby modifying the device behavior such as shifting the wavelength of the laser radiation for a solid-state laser diode.
Large built-in stresses have also been known to appreciably lower the reliability of semiconductor laser diode devices, which frequently heat-up due to the power dissipated during operation, thereby resulting in large thermal stresses developing between the mated semiconductor and metal substrates. The thermal stress can become sufficiently large so as to result in the fracture of the semiconductor substrate and/or the substrates breaking or coming apart due to a failure at the soldered interface over one or more operational cycles.
Additionally, the solders used to join metals to semiconductor substrates containing laser diodes can re-flow from the interface to other areas of the device and/or package, which can result in a number of problems, such as the electrical shorting of the device, the forming of an open circuit condition and/or the solder material encroaching onto the output facet, thereby severely decreasing the amount of laser radiation emanating from the laser diode as well as other problems.
Relatively small temperature increases (few degrees Celsius), can also result in very large decreases in the reliability of laser diodes. Therefore, any phenomena resulting in a slight over-temperature of the semiconductor substrate will have significant and negative effects on the laser diode device reliability.
Soldering processes typically use a flux material to facilitate the soldering. These flux material mixtures are highly corrosive and, as a by-product of the soldering process, some residual flux will be left remaining on the surfaces after the joining process has been completed. This residual flux material can have a negative impact on the performance and reliability of the semiconductor laser diode device(s) and/or the metal substrate.
Consequently, it would be desirable to be able to implement laser diodes and laser diode array systems where these and other disadvantages are eliminated or reduced and this can be achieved by directly bonding a metal substrate to a semiconductor substrate containing laser diodes or laser diode arrays without the use of an intermediate solder layer. Moreover, it would be even more desirable if the bonding between the semiconductor and metal substrates could be performed at low temperatures. Additionally, it would be desirable if the metal substrate can be adjoined to both sides of the semiconductor substrate without the use of a solder.